Present CT2 (or telepoint) systems in accordance with the CT2 Common Air Interface (the regulations governing the CT2 communication systems) have been allocated 4 MHz (864.1-868.1) of spectrum, which provides 40 communications channels spaced at 100 kHz intervals. This common band has been allocated across Europe and in several territories in the far east. However, due to capacity limitations in the CT2 Common Air Interface several jurisdictions may increase the portion of the spectrum allocated to CT2 by an additional band or two additional bands at each side of the present CT2 band.
The CT2 CAI specifications were specifically designed for 40 channels and much of the system timing is determined by this fact. For instance, a calling handset initiating a link request will transmit in accordance with a multiplex protocol known as MUX3 on a selected free channel for a minimum of 750 mSec. This allows a base scanning 40 channels for incoming link requests 750 mSec, to find and respond to the calling handset. In order to identify a link request, the base must first detect a channel marker in the link request signal. The channel marker is a code transmitted every 14 mSec. in the MUX3 protocol. To allow for synthesizer channel lock and for worst-case phase alignment to the MUX3 data when coming on channel, a minimum of 16 mSec. is required to evaluate each channel for link request activity. With this timing, a base can scan 40 channels in 640 mSec. (i.e., 40.times.16) which will allow it to detect a 750 msec. link request on any of the channels. However, with existing timing, if the base were required to scan 60 channels, incoming (handset-to-base) link requests would go undetected. Thus, a method for allocating channels that advantageously utilizes the extended spectrum and that is compatible with existing timing is needed.
Moreover, even when the additional bands are allocated to CT2, many handsets will lack the capability to operate on the additional (extended) bands. Therefore, backward compatibility is desirable.